1. Field of the Invention
This invention relates to improvements in stacked multicell thermal batteries, and more particularly to increasing power and longevity in stacked multicell batteries used under high rate conditions such as in sonobuoy systems.
2. Description of the Prior Art
Future thrusts in the sohobuoy area call for active sonobuoys, i.e., buoys with sufficient power to send out an acoustic signal to a target, receiving the return signal that has bounced from the target and actively searching for the location of the target rather than just listening passively. Because an active search and locate mode is required, active buoys must employ a power source capable of providing high power pulses ranging from several hundred watts to as much as 1 kW or higher. Pulse durations may vary and duty cycles (percent of the time the battery is pulsing) can vary from low values of 10% to conceivably approaching 100%.
Because the both volume and the weight of the sonobuoy must be minimized and the battery itself should be quiet, a thermal type battery is a possible choice. Unfortunately, conventional thermal batteries have certain drawbacks. These drawbacks include: (1) high self-discharge rates which severely limit their operating lifetime to about one hour or less whereas several hours may be required; (2) keeping the batteries at an operating temperature of greater than 350.degree. C. for extended operating periods of one hour or greater can result in thermal decomposition of the cathode material rendering multihour missions difficult and most likely impossible; and (3) the high powered pulses required dictate a system with minimal internal impedance.
Thermal batteries are made of a number of individual cells which may be stacked one above each other to comprise a bipolar stack. These stacks (if more than one are employed) may be connected in parallel or in series. Each cell of the battery has an equivalent voltage associated with it. Stacking the cells in bipolar fashion allows the voltage of the battery to be adjusted by simply employing the appropriate number of bipolar cells in the cell stack. Each cell consists of an anode, a cathode and a separator which separates the anode material from the cathode material. The cell electrodes contain an electrolyte, typically a mixture of alkali metal halide salts, which is a liquid at the battery operating temperature.
As a thermal battery is heated to operating temperature, the electrolyte melts. The melting point of the electrolyte depends upon the particular material used. Below the electrolyte melting point, the battery is benign and no power can be drawn from it. Typically, the lowest melting point electrolyte that is used melts in the range of 310.degree.-320.degree. C. In many applications of thermal type batteries, such as sonobuoys, the battery must become active in a very short period of time.
In order to raise typical thermal batteries to the operating temperature, a pyrotechnic wafer is placed between each cell. The addition of a pyrotechnic wafer between each cell increases the overall length of the cell stack because for every cell a pyrotechnic wafer of substantial thickness is provided. The pyrotechnic is sized and the composition is selected so as to obtain the correct amount of heat required. A typical pyrotechnic formulation is composed of roughly 88% iron powder and roughly 12% potassium perchlorate. An electric fuse strip is connected to the pyrotechnic wafers and when an electrical signal is sent to activate the battery the electrical signal ignites the pyrotechnics and they burn very quickly. The heat from the burning pyrotechnic flows up into the cells that are adjacent to the heat pellets and very rapidly melts the electrolyte and the battery is energized. After the pyrotechnic is burned, an iron biscuit remains between the cells which contributes to the overall cell stack resistance.
Conventional thermal batteries use iron disulfide as the cathode material. Solubility of the iron disulfide in the electrolyte can result in loss of battery performance. When using a lithium bromide-lithium fluoride-potassium chloride eutectic electrolyte, self-discharge and/or thermal decomposition can be rapid and full performance after one hour at the operating temperature is difficult. Use of an all lithium electrolyte, such as lithium chloride-lithium bromide-lithium fluoride which melts at around 450.degree. C. and operate at 480.degree. C., does not result in favorable operating performance. While the higher temperature supports high power, this combination of temperature and electrolyte composition is unfavorable for iron disulfide stability and, therefore, premature loss of battery life occurs.
Dissolution and/or decomposition of the iron disulfide is essentially a loss of active battery material, and thus represents a loss of battery capacity. Soluble iron disulfide or elemental sulfur released via thermal decomposition react with the lithium anode, thus discharging the battery.
Many emerging applications require the battery to operate for over an hour and the iron disulfide in conventional thermal batteries can undergo significant self-discharge and/or thermal decomposition in one hour. To make up for that loss of electrical generating capability, more active material must be built in to the battery. The additional active material adds more weight, more volume and more cost. Also, more active material requires the addition of even more pyrotechnic. Typically, sonobuoys are configured such that the length to diameter ratios that are available do not allow for excessively long battery stacks. Therefore, a battery design that better utilizes the available outer peripheral dimension of the battery volume is needed.